I get mine the same places as Lilly.Prices only vary a coulple of bucks here and there. I have never seen black cords,you might have to paint them.
I have a yard full of rocks,so I get white and/or biege .It seems to blend right in.Also,I have been know to bury them in the dirt temporarily for the season.It does'nt seem to hurt them.
Best solution for safety and not seeing them would be to run them in your trees if you have trees.
Never had to worry about anyone tripping over them.
I would have them at about 20 ft up, which were also used to run the lighting that was attached to the limbs and branches.
Another thought is to have them attached to a GFCI outlet, let's not fry any of your guests!
If you need black, the best and possibly the cheapest solution, may be to make your own. Don't know how much the wire and ends would cost, but would be interesting to see how much each cord comes to pricewise. You should be able to buy the wire and male and female ends at Home Depot or Lowes.
When I had my Party Rental business, I bought a bunch of yellow cords for safety reasons, now I'll have to paint them.
dollar stores, walmart, flea market. I collet them all year round. There really cheap. Also, you dont have to get outdoor cords. (well, unless your using them for more then one night and you plain on them getting wet).
Making cords ( without a commercial account) is VERY cost prohibitive. The wire itself is not that expensive, but the 'ends' are. Unless you are doing a VERY long cable run, buy the cords already put together.
I also bury my cords. I leave them buried for about a month and have had no ill effects from it. Not deep, just under the ground : keeps the kids from tripping. I don't paint the cords or anything like that, so I have orange cords coming out of the ground to plug stuff into.
Watch for sales, anywhere cords are sold. Rule of thumb: you will always need more than you think you will.
Spend a little time on ebay! I find deals on extention cords and all kinds of stuff here and there for great prices. I won a bid on (10) 50' 3 prong cords for $35 with shipping. Try using misspelled words in the search (like extAnsion courds), that's how I find most of my deals.